Electronic package cooling system and heat sink with heat transfer assembly

ABSTRACT

A method for minimizing thermal overhead in an integrated circuit package is described. A heat sink having a base is integrally formed into the package. The base is connected to the die, and a portion of the heat sink projects from the package, forming a post. A heat transfer assembly having a shaft with an aperture is heated until the aperture expands sufficiently to allow the heat transfer assembly to be fitted on the post with a minimum of force. Upon cooling, a tight joint is formed between the heat sink and the heat transfer assembly.

This is a continuation of application Ser. No. 08/470,620 filed Jun. 6,1995 abandoned Jul. 7, 1997 which is a divisional of application Ser.No. 08/186,967 filed Jan. 26, 1994 now U.S. Pat. No. 5,461,766.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The methods and apparatus of the present invention relate generally tothe field of heat dissipation for an integrated circuit (IC). Moreparticularly, the present invention presents a method for providing highefficiency thermal interfaces in an IC package.

2. Art Background

Integrated circuits consume power during operation, generating heat.This heat must be carried away from the IC at a sufficient rate suchthat the temperature of the die remains within operating parameters.Both the operating temperature of the die and the number of wattsdissipated are typically specified for each IC. For example, a typicaldie operating temperature is 95° C. Failure to cool the IC properly canlead to the IC failing during operation.

A typical prior art solution uses a heat sink that is attached to the ICpackage in conjunction with an air-moving cooling system. A fan blowsair onto the heat sink, transferring heat from the heat sink to the air.The fan causes the heated air to be carried away from the heat sink. Thecooling requirements of the cooling system may be expressed by thefollowing equation:

    (1) Q=ΔT·C.sub.p ·M

Here, Q is the number of watts to be dissipated; ΔT is the difference intemperature between the heat sink (T_(sink)) and the ambient air(T_(air)); C_(p) is the thermal conductivity of air and is a constant;and M is the mass-flow of the air to be moved by the fan of the coolingsystem.

Typically, the interface between the heat sink and the package is of alow thermal efficiency. For example, a heat sink can be glued onto theoutside of the package. The glue typically is of a higher thermalimpedance than the heat sink. Further, there is some thermal impedancebetween the die and the package. The thermal impedances combine,preventing some heat from reaching the heat sink. The trapped heat orthermal overhead is a function of the number of watts dissipated by theIC and the thermal impedances between the die and the heat sink. Thisthermal overhead appears as a temperature gradient between the die andthe heat sink. Thus, T_(sink) is actually the temperature of the die(T_(die)) minus the thermal overhead (T_(over)). Every low efficiencythermal interface in the package/heat sink system adds to the thermaloverhead.

In a low power device, the thermal overhead is not critical, but thermaloverhead is of great concern in modern high-power ICs. It can be seenthat if the thermal impedances remain the same for the high-powerdevices, T_(over) increases. This is because, for design purposes,T_(die) is a constant equal to the operating temperature of the IC (e.g.95° C.). Thus, T_(sink) decreases, and the fan must provide a greatermass-flow of air in order to cool the IC. Practically speaking, the fansize most probably will be increased for very high-power devices,resulting in a corresponding increase in the noise signature of thecomputer system and greater cost. Such a solution is not desirable.Accordingly, the thermal overhead should be minimized to avoid the needfor larger fans.

Therefore, as will be described, the method and apparatus of the presentinvention provide an improved integrated circuit package having minimalthermal overhead. More specifically, a heat pipe is integrally formed inthe integrated circuit package, and a heat transfer assembly is fittedover the heat pipe using minimal pressure. The thermal interfaces thusprovided are thermally efficient.

SUMMARY OF THE INVENTION

An apparatus for minimizing thermal overhead in an integrated circuitpackage is disclosed. The integrated circuit package contains a die thatgenerates heat. A heat sink having a base is provided. The heat sink maybe a heat pipe or a solid piece of thermally conductive material. Thebase of the heat sink is connected to the die such that a projectingportion of the heat sink projects from the integrated circuit package.The projecting portion of the heat sink forms a post upon which a heattransfer assembly can be fitted.

A heat transfer assembly is provided. The heat transfer assembly can bea fin assembly and includes a shaft having an aperture. The heattransfer assembly is heated in order to expand the aperture so that itmay be fitted on the post while applying minimum to no pressure. Theaperture is fitted on the post, and the heat transfer assembly isallowed to cool. The compressive forces of the cooling material of theheat transfer assembly create a tight joint between the heat sink andthe heat transfer assembly. Thus, themal overhead is reduced.

BRIEF DESCRIPTION OF THE DRAWINGS

The objects, features, and advantages of the method and apparatus of thepresent invention will be apparent from the following detaileddescription of the invention in which:

FIG. 1 is a perspective view of an illustrative embodiment of the ICpackage design.

FIG. 2 is a cross-sectional view of the illustrative embodiment.

FIG. 3 is a perspective view of a heat transfer assembly.

FIG. 4 illustrates the manner in which the heat transfer assembly is fitover the heat pipe of the IC package design.

FIG. 5 is perspective view of the IC package with the heat transferassembly attached in an computer illustrative system..

DETAILED DESCRIPTION OF THE INVENTION

An IC package has a heat sink as an integral part of the package. Theheat sink projects from the surface of the package forming a post. Afterthe package is assembled, a heat transfer element having an aperture ismounted on the post by first heating the heat transfer assembly untilthe aperture expands sufficiently to slide easily over the post.Compressive forces upon cooling ensures an excellent thermal jointbetween the heat spreader and the heat transfer assembly, reducing thethermal overhead for the IC. Although the present invention is describedwith reference to specific figures, it will be appreciated by one ofordinary skill in the art that such details are disclosed simply toprovide a more thorough understanding of the present invention. It willtherefore be apparent to one skilled in the art that the presentinvention may be practiced without the specific details.

FIG. 1 is a perspective view of an exemplary IC package 100 having aheat sink 110 as an integral part of the package. Heat sink 110 may bemanufactured of a thermally conducting material such as copper. The heatsink 110 can be a heat spreader or a heat pipe and is preferably acylinder having its top and side surfaces ground to a smooth evenfinish. Finishing the surfaces of the cylinder helps to ensure that theheat transfer assembly will make good thermal contact with the heat sink110. The height of the heat sink 110 is preferably such that typicalmanufacturing robotics need not be redesigned to handle the package 100.It will be understood by one having ordinary skill in the art that theshape of the heat sink is not limited to a cylinder. For example, theheat sink can be configured as a multiple pin assembly, rather than as asingle cylinder. To ensure good thermal contact for all heat sinkshapes, the shape of the heat transfer assembly should be altered toaccount for changes in the shape and configuration of the heat sink.

FIG. 2 is a cross-sectional view of the IC package of FIG. 1. It can beseen that the heat sink 110 is thermally connected to the heat sink base115 which is thermally connected to die 120. Because the heat sink is anintegral part of the package, that portion of the thermal overheadassociated with attaching the heat sink to the package 100 is minimized.Assembly of the package can be done according to methods known in theart.

FIG. 3 is an exemplary heat transfer assembly. In the illustrativeembodiment, the heat transfer assembly is a fin assembly 300 having aplurality of fins 310 and a shaft 320. The shaft 320 has an aperture forreceiving the post formed by the heat sink (not shown). At roomtemperature, the diameter of the aperture of the shaft 320 is preferablyless than or equal to the diameter of the heat sink shown in FIGS. 1 and2. Controlling the size of the aperture of the heat transfer assemblyrelative to the heat sink is important to achieving good thermalcontact. The fin assembly 300 is preferably manufactured of copper orsome other material of high thermal conductance. In order to fit the finassembly 300 on the heat sink, the fin assembly 300 is heated toapproximately 400° C. which causes the diameter of the aperture toexpand by approximately 0.005 inches. This allows the heated finassembly 300 to be dropped onto the heat sink, which is at roomtemperature, minimizing the amount of pressure applied to the package.The ease of fitting the heat transfer assembly on the heat sink isimportant because the heat sink is in direct contact with the die.Excess pressure placed on the heat sink can result in the die, the bondwires, or the package itself being damaged.

As the fin assembly 300 cools, the diameter of the aperture shrinks. Atight thermal joint is ensured by the compressive forces. Because thejoint is a tight connection between two materials of high thermalconductance, the thermal overhead is reduced. This method is simple toimplement and may easily be performed by existing pick-and-placesystems. For example, the heat transfer assembly may be heated in atemperature controlled oven. The package having the integral heat sinkis held in a fixture, and the heated heat transfer assembly is droppedonto the package such that it fits over the shaft of the heat sink. Theentire assembly is then cooled to room temperature with a blast of coolair. Immediate cooling avoids heating the package and its die attachabove its melting or degradation value. FIG. 4 illustrates how the heattransfer assembly 300 can be fitted over the heat pipe 110.

FIG. 5 is a perspective view of a cooling system for a computer systemthat implements an illustrative integrated circuit cooling system. Thecooling system 400 includes the integrated circuit package 410 with afin assembly 420 attached. During normal operation, the die in theintegrated circuit package 410 generates heat which is transferred tothe heat sink. The fin assembly 420 is tightly coupled to the heat sink,and heat is transferred to the fin assembly 420. The fan 430 blows coolair over the integrated circuit package 410 and the fin assembly 420.Heat is transferred from the fin assembly 420 to the air, heating it.The heated air is carried away from the integrated circuit package 410and the fin assembly 420, as indicated by air flow lines 450.

Thus, method and apparatus of the present invention that provide animproved integrated circuit package having minimal thermal overhead havebeen described. While the method and apparatus of the present inventionhave been disclosed in terms of the presently preferred and alternativeembodiments, those skilled in the art will recognize that the presentinvention may be practiced with modification and alteration withoutdeparting from the spirit and scope of the invention. The specificationsand drawings are, accordingly, to be regarded as illustrative, notrestrictive.

What is claimed is:
 1. An integrated circuit package cooling systemcomprising:a package, comprising:a die having an integrated circuitformed thereon, and a heat sink having a first end and a second end, aheat sink base defining the first end and being directly connected tothe dies and the second end projecting from the package to form a post;and a heat transfer assembly compressively attached to the post, whereinthe heat transfer assembly is heated in order to be fitted on the postwith a minimum pressure and subsequently cooled to form a tight jointwith the post for all operating temperatures of the integrated circuit.2. The cooling system of claim 1, wherein the heat transfer is a finassembly.
 3. The integrated circuit package cooling system of claim 1,wherein the heat sink is a heat pipe.
 4. In a computer system, a coolingsystem comprising:a fan that blows air; a package, comprising:a diehaving an integrated circuit formed thereon, and a heat sink having afirst end and a second end, a heat sink base defining the first end andbeing directly connected, to the die and the second end projecting fromthe package to form a post; and a heat transfer assembly compressivelyattached to the post, wherein the heat transfer assembly is heated inorder to be fitted on the post with a minimum pressure and subsequentlycooled to form a tight joint with the post for all operatingtemperatures of the integrated circuit.
 5. The cooling system of claim4, wherein the heat sink is a heat pipe.
 6. The cooling system of claim4, wherein the heat transfer assembly is a fin assembly.